A semiconductor device fabricating procedure includes a thin film deposition process, such as a CVD process (chemical vapor deposition process), for forming a thin film, such as a silicon dioxide film, a silicon nitride film or the like, on a workpiece, such as a semiconductor wafer. Such a thin film deposition process uses a thermal processing apparatus as shown in FIG. 15 to form a thin film on a semiconductor wafer by the following procedure.
A heater 53 heats a double-wall reaction tube 52 consisting of an inner tube 52a and an outer tube 52b at a predetermined temperature. A wafer boat 55 holding a plurality of semiconductor wafers 54 is loaded into the reaction tube 52 (inner tube 52a). Gases in the reaction tube 52 are discharged through an exhaust port 56 to set the interior of the reaction tube 52 at a predetermined reduced pressure. After the interior of the reaction tube 52 has been set at the predetermined reduced pressure, process gases are supplied through a gas supply pipe 57 into the inner tube 52a. The process gases undergo a thermal reaction, and a reaction product produced by the thermal reaction deposits on the surfaces of the semiconductor wafers 54 to form thin films on the surfaces of the semiconductor wafers 54.
Waste gases produced by the thin film deposition process are discharged outside the thermal processing unit 51 through an exhaust pipe 58 connected to the exhaust port 56. A trap and a scrubber and such, not shown, are placed in the exhaust pipe 58. The trap removes the reaction product and other substances contained in the waste gases to discharge the waste gases from the thermal processing unit 51 after rendering the waste gases harmless.
The reaction product produced by the thin film deposition process deposits not only on (adheres not only to) the surfaces of the semiconductor wafers 54, but also on (but also to) the inner surfaces of the thermal processing unit 51 including the inner surface of the inner tube 52a and the surfaces of jigs. If the thin film deposition process is continued in the thermal processing unit 51 with the reaction product adhering to the inside surfaces of the thermal processing unit 51, the reaction product will eventually come off and produce particles. The particles contaminate the semiconductor wafers 54, which reduces the yield of semiconductor devices which are manufactured using the semiconductor wafers 54 contaminated with those particles.
To avoid such troubles, a cleaning process is carried out after the thin film deposition process has been repeated several times to clean the thermal processing unit 51. The cleaning process heats the reaction tube 52 at a predetermined temperature by the heater 53, supplies a cleaning gas, such as a fluoride gas, into the heated reaction tube 52 to remove by etching the reaction product adhering to the inner surfaces of the thermal processing unit 51.
The fluoride gas for such a purpose is a perfluorocompound, such as CF4, C2F6, NF3 or SF6. Generally, the perfluorocompond has a long life. For example, CF4 lasts 50,000 years or longer. The emission of the perfluorocompound into the atmosphere causes global warming. Since there is the possibility that the use of the fluoride gas as a cleaning gas negatively affects the global environment, studies have been made to use a cleaning gas other than the perfluorocompound, such as fluorine gas (F2).
The interior of the reaction tube 52 must be heated at the predetermined temperature to make the cleaning gas etch the reaction product at a desired etch rate to remove the reaction product deposited on the inner surfaces of the thermal processing unit 51. The interior of the reaction tube 52 needs to be heated at a high temperature of, for example, 400° C., to etch the deposited reaction product at a desired etch rate using fluorine gas as a cleaning gas.
If the interior of the reaction tube 52 is heated at such a high temperature of 400° C., the reaction tube 52 formed of quartz and the jigs formed of silicon carbide (SiC) are etched at etch rates higher than that at which the reaction product is etched, and reaction product selectivity, i.e., the ratio between etch rates for the reaction product and the material, decreases. Consequently, the reaction tube 52 formed of quartz, and the jigs formed of SiC are deteriorated when the reaction deposit is removed.